Aircraft attitude indicators of the general type to which this invention relates are disclosed in U.S. Pat. Nos. 2,779,101 to Chombard and 3,516,055 to Snider. Attitude instruments embodying the principles disclosed in those patents are in substantial commercial use.
In certain of its forms such an attitude indicator has a spherical display that is gyro controlled so that regularly spaced meridians delineated on the spherical display are maintained in a vertical orientation in space and regularly spaced parallels that are transverse to the meridians are correspondingly maintained horizontal.
Such a prior display is illustrated in FIG. 1. One of the delineated parallels is an equator that divides the displayed sphere into upper and lower hemispheres and corresponds to the horizon. Relatively fixed to the aircraft in substantially centered relation to the display is an indicator or reference that symbolizes the carrying aircraft and relates its attitude to the attitude of the delineated meridians and parallels on the displayed sphere. Typically this reference takes the form of a circle or dot that symbolizes the fuselage of the carrying aircraft to denote pitch and heading attitudes together with aligned horizontal dashes or bars at opposite sides of the circle or dot that symbolize the wings and show roll or banking attitude. In level flight the airplane symbol is on the equator delineation of the spherical display, and attitudes of climb or dive are indicated by departures of the equator from that symbol, which departures correspond in direction and magnitude to the direction and magnitude of departure of the longitudinal axis of the aircraft from its normal level flight attitude. In like manner, roll attitudes are depicted by the angle between the equator and the wing symbols of the reference.
Thus, the equator of the spherical display provides a horizontal reference to which the pilot relates the attitude of the aircraft in the same way that he relates aircraft attitude to the natural horizon under visual flight conditions. The meridian delineations of the display can be marked in terms of compass headings and can be confined to fixed orientations in space so that the instrument can combine pitch and roll information with information that would otherwise have to be obtained from a separate directional gyro instrument.
In a steep dive or climb under visual flight conditions the pilot cannot see the horizon by looking forward but must orient himself by looking to one side or the other to find the horizon. However, the presentation on a conventional spherical display attitude indicator is analogous to what the pilot could see if restricted to forward vision under visual flight conditions. Consequently there are ranges of diving and climbing attitudes in which the equator line of the attitude display is only partly visible at one side of the display, or is not visible at all. What this means, in practical effect, is that the heretofore conventional attitude indicator gives the pilot the least information about the attitude of his aircraft under precisely those conditions of maneuvering flight in which he most needs to have information that will orient him.
Stated very simply, a conventional attitude indicator tells the pilot where the horizon is, but in steeply climbing and steeply diving flight it does not clearly tell him which way is "up" and thus leaves him without the information that he most needs for recovery from his abnormal attitude. Of course "up" and "down" information is not critically important in a steep climb, but it is absolutely essential for successful recovery from a steep dive. The failure of heretofore conventional attitude indicators to provide such information has been particularly troublesome in maneuvers that combined roll with a diving angle of more than 45.degree.. In fact, investigation has shown that crashes and near-accidents have resulted from misinterpretation of the information presented by prior attitude indicators, which is to say that such indicators did not clearly and unambiguously present the information most needed, or did not present that information at all.
From the fact that the above mentioned Chambord patent issued in 1957, and the further fact that losses of life and of aircraft are known to have resulted from the deficiencies of prior attitude indicating instruments, it is apparent that the solution to the problem posed by such indicators has not been obvious to those skilled in the art.